They had no choice. If they didn't fight they were executed. The higher-ups created a system to stop truces like these so that the war would go on. For example small groups would be forced out to the enemy trench. If they didn't return they were dead. If they all returned empty handed they hadn't been doing their job (capturing enemies). That way it was obvious if they were just pacively laying in no man's land for a while or if they actually were doing their jobs.
Although those fighting men were brave enough to endure that hell, I think that it's a shame they were not brave enough (on both sides) to refuse to go back to their respective trenches and start fighting again.
You are ignorant of the laws of war leaving the trenches would be desertion and depending on your state you could compound that with treason and once again depending on the nation state could face execution
Actually I heard from somewhere that apparently one of the sides had their Christmas presents stolen, which might've been why the truce ended.
So that sucks.
So that sucks.